Here's the definition:
Hail is a large frozen raindrop produced by intense thunderstorms,
where snow and rain can coexist in the central updraft. As the
snowflakes fall, liquid water freezes onto them forming ice pellets
that will continue to grow as more and more droplets are
accumulated. Upon reaching the bottom of the cloud, some of the ice
pellets are carried by the updraft back up to the top of the
storm.
As the ice pellets once again fall through the cloud, another layer
of ice is added and the hail stone grows even larger. Typically the
stronger the updraft, the more times a hail stone repeats this
cycle and consequently, the larger it grows. Once the hail stone
becomes too heavy to be supported by the updraft, it falls out of
the cloud toward the surface. The hail stone reaches the ground as
ice since it is not in the warm air below the thunderstorm long
enough to melt before reaching the ground.
